Hand-held electric field imager for measuring the electric field in mammalian skin and other epithelial structures

ABSTRACT

The present invention provides a hand-held, noninvasive diagnostic device for measuring the electric fields in mammalian skin and other epithelial structures. The device includes an outer housing that contacts the skin, providing stability and allowing the device to move along with minor movement of the skin. Recessed within the outer housing is a probe that acts as a sensor to measure the electric field in the skin through an aperture in the bottom surface of the outer housing. By applying a series of known voltages while the probe is vibrating, the skin&#39;s local surface potential can be measured and the lateral electric field can be calculated from the spatial distribution of surface potential measurements. Active feedback is used to maintain a constant distance between the probe and the skin surface.

FIELD OF THE INVENTION

This application is directed to a device for acquiring information from skin and other epithelia, and particularly to a hand-held device for measuring the electric field of skin or epithelial structures in a clinical setting.

BACKGROUND OF THE INVENTION

Electric fields exist in many forms within the bodies of mammals, such as the electrocardiogram of the heart and the electroencephalogram of the brain. In addition to these more well-known fields, electric fields are also present in the epithelium, which is a tissue composed of layers of cells that line the cavities and surfaces of organs throughout the body, as well as forming part of the skin itself. The epidermis of the skin is composed of stratified squamous epithelial cells and normally generates a voltage across itself called the transepithelial potential. This voltage difference is generated by pumping positive ions from the apical side to the basal side of the epidermis.

FIG. 1A shows a typical epithelial cell with the structures that generate the transepithelial potential. The cell exhibits a polarized distribution of Na⁺ and K⁺ channels, where the Na⁺ channels are concentrated on the apical side of the epithelial cell and K⁺ channels are on the basal side. Utilizing a Na³⁰ /K⁺-ATPase to lower intracellular [Na³⁰ ] and raise intracellular [K³⁰ ] results in a flow of positive ions across the epithelium. The low intracellular [Na³⁰ ], combined with the negative membrane potential, results in Na³⁰ movement into the cell on the apical side where the channels are localized. Similarly, the high intracellular [K³⁰ ] results in K³⁰ efflux on the basal side where the K³⁰ channels are localized. This transepithelial ion flux creates a transepithelial potential of between approximately 20-55 mV that is positive on the basal side (inside) of the epithelial layer (FIG. 1B). In the epidermis, current flow is limited by the very high resistance of the stratum corneum and the tight junctions between epidermal cells that form the multilayers composing the epidermis.

When the skin is wounded, such as by a cut, the ionic currents exit the skin at the site of the wound, driven by the voltage generated across the epidermis. As shown in FIG. 1C, after wounding, the transepidermal voltage will immediately drive current out of the low resistance pathway created by the wound, generating a lateral electric field from the flow of wound current on both sides of the epidermis. The field is generated because the positive wound current flows toward the wound on the basal side of the epidermis and flows away from the wound on the apical side. Accordingly, the two sides of the epidermis at the wound site will exhibit opposite polarities: there will be a positive pole at the surface and a negative pole deeper in the epidermal multilayer.

These electric fields are important to wound healing; for example, various studies have shown that the endogenous electrical field near the wound directs epithelial cell migration to improve healing. Manipulation of electric fields near the wound could have direct application in enhancing wound healing, but because there is a lack of reliable information regarding the electric fields associated with normally healing wounds, in humans, no consistent methodology has been established for the use of electric fields in such treatment. To formulate comprehensive treatments, the polarity and magnitude of the endogenous wound current within and directly adjacent to the wound must be determined. In addition, a demonstration that endogenous wound fields are attenuated in chronic wounds would also be necessary. Thus far, the conventional techniques for determining this information have been limited in their usefulness.

Conventional techniques offer two methods for obtaining measurements of the electric fields. First, a direct method involves inserting electrodes into the skin to measure the voltage differences. This method has several disadvantages including risk to the subject from broken or damaged electrodes, difficulty positioning the electrodes, and interference from movement of the subject. In addition, to reduce noise, the measurement setup must be placed in an electromagnetically shielded cage, which severely hampers the portability and ultimate patient utility of the measurement system.

A variation on this direct method uses skin surface electrodes, however this approach is problematic because the electrodes are placed on the highly resistive stratum corneum while the signal that they must detect is beneath that layer at the stratum granulosum. The resistance of the stratum corneum varies from day to day and in different body locations and emotional states, making it difficult to reliably measure the very small potential variations (several millivolts) over the small distances (on the order of 100 μm) involved here.

Second, an indirect method for measuring electric fields in skin utilizes a Kelvin probe, which was originally designed to measure the work function of various metals. A Kelvin probe functions by creating a parallel plate capacitor out of the probe head and the surface being studied. The work function of the surface being studied can be measured quickly by regulating a biasing voltage applied to either the probe or the surface. Although effective for measuring the surface potential of various plant materials, the Kelvin probe has several problems when applied to mammals.

Primarily, the movement of mammals, unlike relatively immobile plants, makes it difficult to obtain accurate measurements using a Kelvin probe. While anesthesia can be used to immobilize a subject, its use poses unacceptable health risks for humans. Even when a subject is not overtly moving, mammalian skin is pliable and subject to constant movement due to respiration and circulation within the body. In addition, signal artifacts are common when measuring mammalian skin from sources such as hair, which has a substantial static charge that influences surface potential readings, and interstitial fluids, which often fill wounds and have a different work function than the surrounding skin that influences the measurement of the electric field.

A particularly good system for overcoming many of the problems with the conventional techniques is presented in the co-pending patent application entitled “Application of the Kelvin Probe Technique to Mammalian Skin and Other Epithelial Structures,” (U.S. patent application Ser. No. 11/031,188), incorporated herein by reference. That application describes a Bioelectric Field Imager (BFI) where a probe detects electric fields in the skin without contacting the region being studied by forming a parallel-plate capacitor between the skin and a sensor tip, then vibrating the sensor tip and taking measurements to determine the electric field. Although the BFI is a very effective system, it is a bench-mounted device and is designed to perform scans in the x-y plane on horizontal, motionless surfaces, which generally requires that subjects be placed under anesthesia. Because of the risks associated with this requirement, the BFI is not suited for general, routine use on human subjects. In addition, because of the physical constraints, the BFI is not ideal for use in medical offices or other outpatient settings.

Accordingly, there is a need for a system that overcomes the problems in the conventional techniques and provides additional features that make measuring the electric field in mammalian skin easier and more convenient. It would be desirable to have a system that measures the electric field non-invasively and without the use of anesthesia to immobilize the subject. It would also be desirable to have a noninvasive system that is hand-held and can be easily manipulated to contact surfaces at a variety of orientations. The hand-held, noninvasive system would preferably adapt to the small continuous motions of mammals and be suitable for monitoring wound healing and for examining skin features such as wrinkles.

SUMMARY OF THE INVENTION

The present invention provides a hand-held, noninvasive diagnostic device for measuring the electric fields in mammalian skin and other epithelial structures. An embodiment of the invention provides a device that measures the electric fields in the skin of a subject at any angle while minimizing the risk and discomfort to the subject. In another embodiment of the invention, the device evaluates the electric field surrounding a wound or skin lesion. In further embodiments, the device monitors healing of a wound in the skin or examines features of the skin, such as wrinkles.

The device includes a probe that acts as a sensor to measure the electric field in the skin. The probe is positioned inside an outer housing, with the housing placed in contact with the surface of the skin to be examined and the probe recessed within the housing such that the probe does not contact the skin. A vibration unit is coupled to the probe and causes the probe to vibrate in the direction roughly perpendicular to the surface being examined. Vibration units that can produce high speed vibration, such as piezoelectric disks or electromagnetic speakers that produce frequencies of 800 to 1200 Hz, are preferred because the higher vibration frequencies produce stronger signals. The amplitude of the vibration (defined as half of the total vertical displacement of the probe) is preferably 90 μm or less, and more preferably in the range of 20-90 μm.

The probe comprises a conductive metallic tip that forms a parallel-plate capacitor with the skin surface. If the surface potential of the metal piece is different from the surface potential of the skin near it, there will be a build-up of charge on the “plates” of the capacitor. A microcontroller applies a series of known reference voltages (V_(b)), preferably ±5-10 V, to the metal probe or to the skin. The applied voltages induce a flow of charge between the two surfaces when they are connected. Because the probe is vibrating, which varies the capacitance, the flow of charge, or current, oscillates. The oscillating current is measured by a meter in the probe tip and then immediately converted to an oscillating voltage, which is transmitted to the microcontroller. From the oscillating voltage signal, the peak-to-peak voltage values are used to determine the voltage value at which there is no current flow between the two surfaces, which will be equal to the surface potential of the skin at that point. The microcontroller determines the peak-to-peak voltage values in hardware using either analog integration or a peak detector, rather than in software as in the BFI device, which reduces noise spikes and allows faster data acquisition.

After determining the surface potential at several points in a given region, the electric field between any two points is given by the difference in surface potential at these points divided by the distance between them. Accordingly, the probe eliminates the need to use penetrating electrodes or contact electrodes to measure an electric field, and makes electromagnetic shielding unnecessary.

The outer housing in which the probe is positioned is held in contact with the skin surface being examined to provide stable positioning for the probe, keeping the probe in the same frame of reference as that skin surface such that the probe stays in approximately the same position and orientation with respect to the skin. Because the outer housing rests on the skin surface, it moves with the skin, allowing the entire device to move with the skin as well. The outer housing includes an optically transparent lens as its bottom surface, which is the surface in contact with the skin. The lens has an aperture, or opening, over which the probe is positioned and through which the measurements are taken.

The aperture is preferably a narrow slit, most preferably about 1 mm wide, because this size and shape causes a minimal amount of protrusion of the skin into the aperture. It is important to minimize protrusion of the skin into the aperture to provide a substantially flat skin surface for accurate measurements, as further discussed below. Other shapes may be used for the aperture, particularly when examining skin that is relatively taut and not inclined to protrude up into the aperture. The aperture may also be covered with an electrically transparent material, which is a material that is non-conductive and does not interfere with the electric field, such as polyethylene or other polyvinyls. Covering the aperture is helpful when measuring the field at a wound site where any kind of fluid is present, such as blood or interstitial fluid, to avoid artifacts that interfere with accurate electric field measurements.

Two stepper motors control the positioning of the probe in relation to the skin being examined. The probe is moved parallel to the skin by means of a first miniature stepper motor that moves the probe in increments of 40 μm within the housing from one end of the aperture to the other. The movement allows the probe to scan the region of skin exposed by the aperture to obtain the measurements necessary to determine the electric field for that region.

At the same time, a second miniature stepper motor controls the distance between the probe and the skin in the direction perpendicular to the skin surface (generally referred to as the “z” direction in this context). While the outer housing of the device provides stable positioning for the probe, as discussed above, the sensitivity of the capacitor requires substantial precision because the capacitance of the parallel-plate capacitor formed by the two surfaces is highly dependent on the amount of separation. Thus, the second stepper motor is required to provide this fine-tuned control mechanism. The second stepper motor continually adjusts the probe to maintain a constant distance between the probe and the skin in response to minor motions of the skin and to variations in the topography of the skin over the region scanned.

Because of the importance of taking each measurement at the same distance from the epithelium being investigated, it is necessary to maintain a constant separation distance between the probe and the skin. Although it might appear contradictory to attempt to maintain this constant distance when the probe's position in the z direction is continually changing due to its vibration, it is possible to control the separation such that any given point in the path of the oscillation remains at the same distance from the epithelium. Thus, a constant distance is maintained by selecting a point in the oscillation path of the probe and maintaining that point at the same distance from the epithelium for all measurements. Here, the second stepper motor maintains the separation distance at which the probe tip is closest to the skin (the “distance of closest approach”) at a constant value. The distance of closest approach is determined from the probe signal using a software algorithm and is used to provide feedback to the second stepper motor. The second stepper motor establishes the probe's position in the z direction prior to taking measurements with the probe. The distance of closest approach is preferably 500 μm or less, more preferably in the range of 20-500 μm, and is preferably maintained to within a tolerance of approximately 6 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1C depict diagrams of mammalian skin and the current that is driven out of the skin by the transepidermal potential when a break in the epidermis occurs or a skin lesion results in a low resistance pathway.

FIG. 2 depicts a partially transparent perspective view of the vibration unit assembly and sensor head of the device.

FIG. 3 shows a cross-sectional view of the body and connection handle of the device.

FIG. 4 depicts a perspective view of the lens and aperture in accordance with one embodiment of the present invention.

FIG. 5 shows an exploded view of the body and connection handle of the device.

FIGS. 6A and 6B are photographs of the sensor tip mounted to two different vibration units.

FIG. 7A shows a schematic diagram of the electronic elements and connections of the device.

FIG. 7B-7D show three variations on the circuit used to determine the V_(ptp).

FIG. 8 is a flow chart illustrating the feedback control mechanism for the second stepper motor.

FIG. 9 depicts one embodiment of a graphical interface for the present invention used on a computer.

FIGS. 10A-10F depict different lesion types and their corresponding scans acquired with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a device for measuring the electric fields in epithelial tissue that is hand-held, noninvasive and suitable for use on human subjects, particularly for use in outpatient or other clinical settings. FIG. 2 shows a partially transparent perspective view of the hand-held device 8 according to an embodiment of the invention. As can be seen in the figure, there is a push-button actuator 10 at the top of the device attached to, and extending through, positioning handle 12. Positioning handle 12 is designed to be gripped by one hand of an operator in order to position and orient the device, thus it is preferably a spherical shape, but may be any other suitable shape for gripping. The arrangement of the positioning handle 12 with push-button actuator 10 allows an operator to activate and position the device with a single hand by wrapping a finger or fingers around the positioning handle 12 and engaging the push-button actuator 10 with a single finger. At the base of positioning handle 12 is cap 14 that covers and protects the motors 16 and 18, further discussed below.

Connected to the motors is probe 20, which contains sensing and processing electronics for the device, including a current-to-voltage amplifier circuit 62 (see FIG. 7A). Probe 20 is vibrated by a vibration unit 24 that may be either a piezoelectric disk or an electromagnetic speaker, shown in more detail in FIGS. 6A and 6B, respectively. As shown in FIG. 7A, the signal from the probe 20 is sent through a cable 74 to an electronics package 76, which is connected to a computer 52, where a microcontroller 70 and a V_(ptp) determining circuit 66 analyze the capacitance formed between the gold plate 22 at the bottom tip of probe 20 and the epidermis of the subject (not shown in FIG. 2). The length of the connecting cable, which is usually several feet (e.g. a 6-ft. cable), allows the device to be easily positioned on any part of the subject being examined.

The lower portion of the device, including probe 20 and vibration unit 24, is enclosed in an outer housing 26 (FIGS. 2 and 3). Outer housing 26 is joined to cap 14, forming a covering for the body of the device, and is preferably formed from a plastic material. The bottom of outer housing 26 rests on the skin of the subject and this contact provides stable positioning of the device with respect to the skin surface being examined. Using positioning handle 12, the device may be positioned such that outer housing 26 is placed against a body surface having any orientation. In addition, having outer housing 26 in contact with the skin allows the device to move with the skin as the skin moves due to breathing, circulation, movement of the underlying muscles, or other movement. This matched movement permits the outer housing to hold the skin in a relatively stationary position with respect to the probe 20 and maintains the separation between the skin and sensor tip at a roughly constant distance, which allows the probe 20 to be positioned independently of minor involuntary movement by the subject. Of course, the subject should avoid large movements during scanning to allow the outer housing 26 to be held against the skin.

The bottom surface of outer housing 26 comprises a lens 28 that is formed from a transparent material and has an aperture 30 (shown in more detail in FIGS. 3 and 4). Aperture 30 provides access to the skin for the probe 20 to scan and is preferably a slit about 1 mm wide. The preferred narrow width of the aperture 30 optimizes the surface topology for scanning because it minimizes the amount of skin that protrudes up into the aperture. When skin protrudes up into the aperture, it has a convex shape, thus the separation distance between the skin and the probe 20 is not the same for the entire region of skin exposed by the aperture. Variations in the separation distance makes positioning of probe 20 in the z direction difficult, and has an adverse affect on the quality of the measurements taken, as further discussed with respect to the positioning mechanism. Accordingly, the shape and dimensions of the aperture should be selected to minimize protrusion of the skin into the aperture. Thus, when measuring skin that is particularly loose and flexible, a narrow aperture will be preferred. In contrast, when examining a region of skin that is substantially taut, a larger aperture or an aperture of another shape may be acceptable.

When measuring the electric field at a wound site that has any kind of fluid present, such as blood or interstitial fluid, the fluid can interfere with the accurate measurement of the epidermal surface potential by the device due to the differences in the work function characteristics of the fluid in comparison with those of the surrounding tissue. The work function, which is the minimum energy needed to remove an electron from the surface of a material, is very different for dry skin than for fluids. Thus, when moved from a position over dry skin to one over fluid, the probe will detect a large voltage difference that is due to work function differences alone, rather than from the electric field of interest that is generated by the current flow beneath the epidermis. The effect of these differences in work function can be minimized by placing a thin covering 42 (FIG. 3) over the aperture that is an electrically transparent material, such as polyethylene or other polyvinyls. With the covering 42 in place, the surface work function is the same for all positions scanned by the probe, while the underlying electric field is easily detected through the electrically transparent material.

The probe 20 examines the skin exposed by the aperture 30 as its position is controlled by the first stepper motor 16 in the x direction, which follows the length of aperture 30. In a preferred embodiment, adjustments in the y direction are not necessary because of the narrow width of aperture 30 (see FIG. 4). In addition, if measurements of regions to either side of the aperture 30 are required, the device may easily be repositioned such that the aperture 30 exposes those regions for subsequent scans. As shown in FIG. 3, first stepper motor 16 includes a positioning arm 34 that moves probe 20, along with vibration unit 24, incrementally in the x direction. The first stepper motor 16 is electrically connected to and controlled by the computer 52 (FIG. 7A) via signal wires 54 a extending through connection handle 32 (FIG. 3). The first stepper motor 16 is capable of moving with a 40 μm step size and allows the collection of surface potential data over a linear region of the skin.

The second stepper motor 18 is responsible for maintaining the position of probe 20 in the z direction (FIG. 4), which is roughly perpendicular to the skin surface. The outer housing 26 and lens 28 keep the device steady with respect to movements of the skin; however, the device requires that the distance between the closest approach of probe 20 and the skin be precisely maintained in order to accurately determine the electric field of the skin, thus the second stepper motor 18 provides the necessary fine adjustments. The feedback control mechanism employed by the second stepper motor 18 is discussed in further detail below. Vibration of the probe 20, caused by vibration unit 24, also occurs in the z direction, thus second stepper motor 18 controls the distance of closest approach, as defined above.

The vibration units used in the device are preferably either a piezoelectric disk 36 (FIG. 6A) or an electromagnetic speaker 38 (FIG. 6B). The piezoelectric disk is composed of two piezoelectric ceramics, each bonded to a brass disk, that are in turn attached together. The electromagnetic speaker is composed of a speaker element that vibrates on an axis normal to the skin or other epithelium. The vibration unit 24 vibrates probe 20 at 800 Hz or greater. Currently, frequencies in the range of 800 to 1200 Hz are used, but higher frequencies may be used as faster, quieter vibration techniques and devices are developed. In general, the highest frequency available is preferred as higher frequencies generate a stronger signal. The high frequency vibration used here represents an almost twenty-fold increase over the vibration frequency used in the bench-mounted BFI, allowing for faster data acquisition and overall shorter scanning times. This is advantageous for the subjects because it minimizes the time that they need to remain relatively still.

FIGS. 3 and 5 show a color video camera 40 located inside connection handle 32 that allows the operator performing the scan to see the area over which the device is positioned. Connection handle 32 is comprised of a camera housing 44, right-hand and left-hand adapter rings 46 and 48, and a two part cylinder portion 50 (FIG. 5). The camera 40 images a view through lens 28, as shown in the upper right portion of the graphical interface depicted in FIG. 9. An LED (78 in FIG. 6B) provides illumination to the area. The camera display allows the device to be precisely positioned over a wound or a skin lesion, which reduces the number of scans that do not cover the intended region. In addition to housing camera 40, connection handle 32 provides a conduit for connection of the device to a computer 52 via cable 74 and the electronics package 76 (FIG. 7A). Signal wires extend into connection handle 32 (FIG. 3) from the motors 16 and 18 (54 a and 54 b in FIG. 7A), the vibration unit 24 (56 in FIG. 7A) and the probe 20 (58 in FIG. 7A).

Returning to FIG. 3, probe 20 has an outer portion 60 that forms a conductive metal layer that electrically shields the electronics inside and the gold plate 22 at the bottom. The gold plate 22 and the subject's epidermis (not shown in FIG. 3) form plates of a parallel-plate capacitor. Because the surface potential of each plate is different, charges will build up on the plates and a voltage will develop across the capacitor (the contact potential). A series of reference (biasing) voltages of ±5-10 V, preferably a ±10 V, are applied to either the skin or to the gold plate 22, which causes the charges on the plates to change, meaning that a current flows from one plate to the other via the connection formed across the applied voltage. In a preferred embodiment, the reference voltages are applied to the skin via a conventional skin surface electrode (such as those used in electrocardiography). Application of the reference voltage to the skin results in fewer switching artifacts compared with application to the probe, which allows data acquisition to begin more quickly.

The reference voltages are applied in pairs (e.g. +10 V and −10 V) in an alternating sequence during the measurement period. At the same time, the probe 20 is vibrated in the z direction by vibration unit 24, changing the distance separating the plates of the capacitor. The capacitance of a parallel-plate capacitor depends on the distance between the plates, thus the capacitance is very sensitive to changes in that distance. As the capacitance changes, the charge on the plates is also changed in accordance with the equation Q=CV (here, voltage can be assumed to be relatively constant). The current corresponds with the change in charge over time, given by i=dQ/dt, thus the combination of the applied voltage and vibration of the probe induce an oscillating current.

In one embodiment of the present invention, the oscillating current is measured by the probe 20 and immediately converted to a voltage via a current-to-voltage or transimpedance amplifier 62 (FIG. 7A). To reduce the input capacitance of the current-to-voltage amplifier 62, the gold plate 22 is mounted directly on the amplifier chip and the chip is embedded in plastic and shielded. The output voltage of the amplifier varies periodically as the probe vibrates (an oscillating voltage), and the peak-to-peak voltage (V_(ptp)) depends on the difference between the contact potential and the reference voltage. The signal from the probe 20 is sent via cable 74 to an electronics package 76, where the signal is further amplified through a voltage amplifier 64 to generate an amplitude of approximately 2 V peak-to-peak. The amplified signal is then sent to a circuit 66, where one of two methods may be used to determine the V_(ptp).

In the first method, the oscillating voltage is accumulated over a fixed sample time by an integrator circuit 68 (FIG. 7B). Because the voltage signal is oscillating about 0 volts, it is necessary to invert the signal from negative to positive when the signal is in the negative range in order to integrate. The microcontroller 70 instructs the integrator circuit 68 to invert the signal at the appropriate times in this phase-dependent inversion method. The then-positive signal allows the integrator circuit 68 to sum the total area between the voltage wave and the time axis. Integrator circuit 68 integrates the signal for a fixed number of periods, beginning from the minimum signal level, which is zero in this case, further reducing noise. The integrated signal is proportional to the peak-to-peak value of the vibrating capacitor-induced signal. In an alternative embodiment, the signal may be rectified by an amplifier 80 (e.g. an AD8037, a wide bandwidth, low distortion clamping amplifier) before integration, such that the negative portions of the signal are reflected about the time axis (x-axis) and made-positive (FIG. 7C). However, the rectified signal tends to have a lower signal-to-noise ratio compared to the signal produced using phase-dependent inversion.

In the second method, a peak detector circuit 82 averages the positive peak signals and the negative peak signals (FIG. 7D). This method is less sensitive to slight changes in frequency than the integration method, while providing a high signal-to-noise ratio.

The output of the integrator circuit 68 or peak detector 82 is sensed by the microcontroller 70 via an analog-to-digital converter (ADC) 72 (FIG. 7A). Datasets are measured and transmitted from the microcontroller 70 to the computer 52 at a rate of approximately 21 Hz. Each dataset consists of a data value and integration time versus each applied reference voltage (e.g. a data value and integration time for the +10 V reference and a data value and integration time for the −10 V reference). From the datasets, the average V_(ptp) for each reference voltage is calculated and plotted against the reference voltage values. The resulting line will intercept the reference-voltage axis at-a point that corresponds to the voltage at which there would be no current flow between the two surfaces, which also provides the voltage created by the electric field of the subject's skin at the location measured. As described above, after determining the surface potential at several points in a given region, the electric field between any two points is given by the difference in surface potential at these points divided by the distance between them.

In addition, the line of V_(ptp) plotted against the reference voltages is used in the control of the positioning of the probe 20 in the z direction by second stepping motor 18. The slope of the line is inversely proportional to the distance between the closest approach of the probe and the skin surface, thus by maintaining the slope, the distance is also maintained. The computer 52 uses the slope data to send feedback signals to the second stepper motor 18 via the microcontroller 70. When the slope varies from a target slope value, the computer 52 provides the slope information to microcontroller 70, which generates a control signal for the second stepper motor 18 to adjust the z position of probe 20 before each measurement. The data sampling rate of 21 Hz allows the microcontroller 70 to provide the control signal to the second stepper motor 18 at a rate of 1-5 Hz, as a proportional controller. The amount of adjustment of the probe's position is based on the slope value at that time compared to the target slope value. The steeper the slope of the line, the more change will result from a given adjustment of the probe. Accordingly, the computer 52 continually monitors the separation distance as it evaluates new datasets and produces the line of V_(ptp) plotted against the reference voltages.

As shown in more detail in the flowchart of FIG. 8, the feedback control mechanism begins with the acquisition of the calculated average V_(ptp) values 100. As described above, these values are then plotted against their corresponding reference voltages 102. The slope of the voltage line is determined 104 and compared with a target slope value 106. The feedback control mechanism next determines if the slope is within the tolerance range 108. If it is in the tolerance range, the slope is monitored for a predetermined amount of time 110 to ensure that it remains in the range. If it remains in the range as evaluated at step 112, then the slope can be accepted as a data point 114 and measurements may be taken in order to determine the electric field of the epithelium. As stated above, the position of the probe in the z direction must be established prior to taking any measurements. If the slope does not remain within the tolerance range for the specified period, then the process begins again at step 100 as new V_(ptp) values are acquired.

If the slope was determined not to be in the tolerance range at step 108, then the microcontroller 70 determines how far the probe needs to be moved 116 and generates the appropriate control signal 118 for the second stepper motor 18. The second stepper motor 18 then moves the probe in accordance with the control signal 120. Next, the microcontroller 70 determines how far the probe is from the target 122 and allows a period of time to elapse based on that determination 124. If the probe is far from the target, a short amount of time is required, but if the probe is close to the target, then the amount of time is longer. This variable waiting time allows the slope to stabilize when it is near the target (where it is most important), but also allows the probe to be moved quickly when it is farther away. After the time period has elapsed, the control mechanism returns to step 100 to acquire new V_(ptp) values to continue to evaluate the slope until the positioning of the probe in the z direction is acceptable.

Because the microprocessor 70 generates the control signals for the second stepper motor 18 based on feedback from the probe itself, it is important that the probe's signal contains accurate information about the distance between the probe and the skin surface. The surface topology of the skin can be rather complex near wounds and lesions, thus the distance between the skin and the probe must be adjusted before every measurement to ensure that the distance of closest approach is maintained. The time required for this adjustment can be reduced by flattening the skin to be scanned as much as possible. A narrow aperture 30 helps to accomplish this because the contact of the outer housing 26 with the skin on either side of the aperture causes that skin to flatten. As a result, the skin that protrudes into the aperture tends to be more uniform in height. The greater the uniformity, the faster the probe can be positioned at the appropriate distance from the skin.

FIG. 9 depicts one embodiment of a graphical interface used on the computer. In the upper right panel a real-time video image from camera 40 of the probe 20 and the epithelium being scanned is displayed. Below the video image, the probe's real-time output is displayed as it scans across a lancet wound. The time course of the averaged surface potential is displayed in the lower left panel below an enlarged image of the epithelium being scanned. The increase in potential present as the device scans over the wound indicates a lateral electric field on either side of the wound region.

FIGS. 10A-10F show scans of different lesion types using an embodiment of the present invention taken prior to the removal and sectioning of the lesion along with a histological section of each. Overlays of the scanned signals onto top-view photographs of each lesion type are also provided. Three of the four malignant lesions were associated with a large electric field while all of the benign lesions exhibited very small electric fields.

In additional embodiments of the invention, a series of measurements are taken in the x direction in order to obtain data in two dimensions, rather than the linear measurements described above. In one embodiment, the probe 20 is replaced by multiple probes in order to provide simultaneous measurements at multiple locations. In another embodiment, the probe 20 is provided with multiple sensors to achieve similar measurements. 

1. A hand-held noninvasive diagnostic device for evaluating an electric field associated with an epithelium of a mammal comprising: an outer housing having a bottom surface that includes an aperture, wherein the bottom surface is adapted to contact the epithelium such that the diagnostic device moves with a movement of the epithelium; a probe recessed within the outer housing and positioned above said aperture comprising a conducting plate; a vibration unit attached to said probe for vibrating said probe over the epithelium of the mammal, wherein the vibration of the probe defines an oscillation path of the probe; a voltage supply for creating a voltage bias between the probe and the epithelium; a positioning device attached to the probe to maintain a constant distance between the epithelium and a point on the oscillation path of the probe; and a meter for measuring the current generated by the vibrating probe.
 2. The device as recited in claim 1, wherein the aperture is a slit.
 3. The device as recited in claim 2, wherein said slit is about 1 mm wide.
 4. The device as recited in claim 1, wherein the aperture is covered by an electrically transparent material.
 5. The device as recited in claim 4, wherein the electrically transparent material comprises a polyvinyl material.
 6. The device as recited in claim 5, wherein the electrically transparent material comprises polyethylene.
 7. The device as recited in claim 1, wherein the point on the oscillation path of the probe is the closest approach of the probe to the epithelium.
 8. The device as recited in claim 1, wherein the constant distance is less than or equal to 500 μm.
 9. The device as recited in claim 1, wherein the constant distance is 20-500 μm.
 10. The device as recited in claim 1, wherein the constant distance is maintained to within a tolerance of about 6 μm.
 11. The device as recited in claim 1, further comprising a microcontroller connected to said positioning device and said meter.
 12. The device as recited in claim 1, wherein the positioning device is capable of laterally moving the probe across an area of the epithelium while maintaining the constant distance between the epithelium and the point on the oscillation path of the probe.
 13. The device as recited in claim 12, wherein the positioning device comprises a first motor that controls lateral movement of the probe and a second motor that maintains the constant distance.
 14. The device as recited in claim 1, wherein said bottom surface comprises a lens through which the epithelium is visible.
 15. The device as recited in claim 14, further comprising a camera that provides a video image of the epithelium through the lens.
 16. The device as recited in claim 1, wherein said vibration unit vibrates said probe at a frequency of about 1200 Hz.
 17. The device as recited in claim 1, wherein said outer housing comprises a plastic material.
 18. A method of measuring the electric field associated with an epithelium of a mammal, comprising: (a) providing a hand-held noninvasive device having a probe positioned inside an outer housing, wherein the outer housing has an aperture in a bottom surface and the probe is positioned over a portion of the aperture; (b) positioning the device such that the outer housing contacts the epithelium and the probe is spaced apart from the epithelium by a separation distance, wherein the probe forms a parallel-plate capacitor with a region of the epithelium to be measured; (c) applying reference voltages to the probe or the epithelium; (d) vibrating the probe with a vibration unit, wherein the vibration of the probe defines an oscillation path of the probe; (e) measuring the current induced by the vibrating probe; (f) calculating a surface potential for the region of the epithelium; (g) moving the probe within the housing to additional portions of the aperture; (h) maintaining the separation distance between a point on the oscillation path of the probe and the epithelium; (i) repeating steps (c) through (h) to obtain additional measurements; and (j) generating electric field data for the regions of the epithelium that were measured.
 19. The method as recited in claim 18, wherein the step of calculating a surface potential for the region of the epithelium comprises: converting the current induced by the vibrating probe to a voltage signal; analyzing the voltage signal to determine peak-to-peak voltage values; and determining a reference voltage at which there would be no current.
 20. The method as recited in claim 19, wherein the step of maintaining the separation distance between the probe and the epithelium comprises: plotting the peak-to-peak voltage values against the applied reference voltages to produce a peak-to-peak voltage line; determining a slope of the peak-to-peak voltage line; generating a control signal based on the slope; transmitting the control signal to a stepper motor to move the probe; and moving the probe in accordance with the control signal. 